The main agent in Roundup, glyphosate, is considered by users as among the safest of agricultural chemicals, but some residents say they worry about its frequent use and doubt the safety claims of the chemical industry.

"I just think it's unnecessary. The risk is too great," said Peggy Kadey, who with her husband started the Kaua'i Network for the Chemically Injured.

"I know a lot of people who are affected by it, and it's just not fair to use it in public places for the sake of some weeds. Let's just weedwhack under the fences."

The herbicide glyphosate has many brand names, the most familiar of which is Roundup. Another glyphosate formulation known as AquaMaster was used to kill the salvinia weed in Lake Wilson in 2003.

Jeff Mikulina, director of the Sierra Club Hawai'i Chapter, said there may be isolated cases of appropriate uses for the product, but it should not be applied along roads and in public parks.

"Just get rid of it," Mikulina said. "We think there are less toxic approaches that could be employed, like designing, so no weed control is needed."

Weed-control experts say they've considered the alternatives and most have concluded that Roundup, when applied properly, is among the safest methods to combat unwanted vegetation.

"We do use Roundup," said Winnie Singeo, acting director of the city's Honolulu Botanical Gardens. "Our people suit up entirely when they apply it, but it's supposed to be safe and there's a real quick re-entry time. As soon as it dries, the public can go back into areas."

The state Department of Transportation tries to limit use of the herbicide in areas it can't reach by mechanical means, said spokesman Scott Ishikawa. Roundup is the department's herbicide of choice for spraying under guardrails and on median strips. It is not used in sensitive areas such as near taro patches in Hana, Maui, near East Moloka'i fishponds and Kaua'i's north-shore streams.

Some residents argue that the precautions are not sufficient, both for health reasons and because of overspray.

"Some people report respiratory problems and skin reactions. I get migraine headaches. If they're spraying the roadsides, in many cases there are no alternative highways," said Diane Koerner of Hawai'i's Coalition for Alternatives to Pesticides. "All you have to do is drive down the highway and the big machines are spraying, and you can't avoid it."

On Kaua'i, after meetings with the Kaua'i Network for the Chemically Injured, state highway crews agreed to post signs when they are working in specific areas to alert drivers that they might encounter spray trucks.

That is not done in much of the rest of the state, even though Roundup manufacturer Monsanto suggests it.

"We recommend you post that you're going to spray that day," said Monsanto toxicologist Donna Farmer, who testified last month at a Kaua'i County Council hearing on herbicide use in parks.

Although the state Department of Transportation tries not to spray when it's windy, organic farmers said the problems persist.

"We get so many calls from infuriated people who say they can see it drifting onto their property. It's a big problem for a lot of our organic growers," said Bari Green, educational program coordinator for the Hawai'i Organic Farmers Association.

A crop exposed to Roundup does not meet organic standards.

Farmer said the company recommends that people with weed issues apply integrated pest-management techniques. Several agencies have used it, and The Nature Conservancy of Hawai'i applies it to weed control in its several natural reserves.

Hawai'i Volcanoes National Park has concluded that herbicide use is one of the best solutions for small infestations of invasive grasses.

"We do find herbicides the most practical tool, but our strategy is to minimize use by using it on weeds before they become widespread. We do tests to look at the minimum amount that is effective. We want to reduce costs and minimize our nontarget effects," said Rhonda Loh, a resource manager with the Big Island park.

Maui County does not spray lawns or landscaping at beach parks because heavy public use means it's difficult to find a time when people aren't present, a parks official said. Herbicides other than Roundup are used at ballparks and playing fields, and less so at small community parks. Signs are posted for several days to alert the public that the area was recently treated.

The Maui Public Works department does use Roundup and another herbicide to control weeds around roadways, guardrails, sidewalks and signs.

Even Mikulina concedes there may be safety concerns that argue for roadside spraying in certain areas: "It's more dangerous for the guys when you're out there weedwhacking when cars are coming by at 60 miles an hour."

Big Island road and park maintenance personnel use it in some areas, but Hawai'i County has had an herbicide-free policy along several test roadways since 1990 and at some county parks since 1992. Kaua'i County, in response to concerns by residents, is experimenting with pesticide-free parks, comparing the cost of manual weed control with the use of herbicides.